The term "acrylic polymers", whenever used in the present specification and in the claims, means the homopolymers and the copolymers of the alkyl esters of the acrylic or methacrylic acid wherein the alkyl group contains from 1 to 8 carbon atoms.
Examples of acrylic or methacrylic acid esters are: methylacrylate, ethylacrylate, isopropylacrylate, butylacrylate, methyl methacrylate, isopropyl methacrylate, sec.butyl methacrylate, ter.butyl methacrylate, etc.
The acrylic polymers can also contain up to 50% by weight of units derived from other monomers containing double bonds such as styrene, alpha-methylstyrene, acrylonitrile, (met)acrylamide, n-alkyl maleimides or aryl maleimides etc., or from double-unsaturation monomers such as, for example, butadiene.
The production of acrylic polymers such as, for example, polymethyl methacrylate (PMMA), is generally obtained by means of a discontinuous process in an aqueous suspension.
The reaction occurs in stirred reactors wherein there are introduced the monomer additioned with a radical starter and the aqueous phase containing a suspending agent, which has the purpose to prevent the caking of the polymer which gradually forms.
The polymerization heat is removed through the aqueous phase, so permitting to control the reaction which proceeds until a practically complete conversion of the monomer has occurred. The polymer in the form of beads is separated by contrifugation from the reaction mixture and then it is washed and dried. The PMMA beads are fed to an extruder in order to obtain granules and plates.
The suspension polymerization technology, owing to the type of utilized additives and to the sequence of involved operations, which cannot be integrated in a continuous process, unavoidably gives rise to polymer contaminations which are accompanied by a worsening of the excellent intrinsic optical properties of the PMMA. Such worsening, which seems to scarcely affect the usual applicative requirements, precludes, conversely, the use of the polymer in emerging sectors such as the one of the optical fibers, laser scanning discs and optical instruments in general. The main causes of the optical pollution of the polymer are:
the suspending agent, which remains incorporated in the polymer in little amounts, giving rise to yellowing and loss of transparency; PA1 the high molecular weight polymer fractions, which are apparent in the manufactured articles in the form of microgels, the formation of which is bound to non-homogeity phenomena of the suspension and to the formation of agglomerates and scales in the reactors; PA1 the foreign particles, which infiltrate in the production flow due to the process discontinuity. PA1 difficulty in stirring, feeding and discharging the reacting mass; PA1 difficulty in dissipating the reaction heat and therefore in thermally controlling the reaction; PA1 acceleration of the polymerization rate due to a decrease in the termination rate of the macroradicals and to the consequent increase in the concentration of active species in the reaction mixture; this phenomenon, which is designated as "gel-effect or self-acceleration", gives rise to a higher heat generation in the unit of time. PA1 a. continuously feeding a polymerization reactor with a reaction mixture containing at least a polymerization adjuvant selected from the solvents of the acrylic polymer, preferably from the C.sub.1 -C.sub.10 alkyl esters of organic acids R--COOH, wherein radical R represents a hydrogen atom or an alkyl radical containing from 1 to 6 carbon atoms; PA1 b. partially converting the reaction mixture at high temperature; and PA1 c. devolatilizing the partially converted polymer at high temperature and preferably reduced pressure.
These contamination causes can be removed by using a bulk polymerization process, as is described, for example, in laid-open Japanese patent applications 59/45,310; 59/48,121; 58/88,701, 58/88,702 and 50/34,071, which provides polymers starting from a homogeneous mixture consisting of a monomer containing little amounts of a radical polymerization starter and of a molecular weight modifier, without using other auxiliary materials (for example dispersants). Such system permits to integrate in a single continuous process all the steps, from the ones preceding the polymerization up to extrusion of the polymer in the form of granules or plates, thereby minimizing the risks of infiltrations of foreign particles and sensibly reducing the operating costs.
The obstacles to be overcome for conducting a continuous bulk polymerization process are mainly due to an increase in the reaction mixture viscosity as the monomer is converted into polymer, what involves: